The present invention relates to an implant to be placed into the intervertebral space left after the removal of a damaged spinal disc. Specifically, the invention concerns an osteogenic fusion device that enhances arthrodesis or fusion between adjacent vertebrae while also maintaining the normal spinal anatomy at the instrumented vertebral level.
In many cases, low back pain originates from damages or defects in the spinal disc between adjacent vertebrae. The disc can be herniated or can be affected by a variety of degenerative conditions. In many cases. these pathologies affecting the spinal disc can disrupt the normal anatomical function of the disc. In some cases, this disruption is significant enough that surgical intervention is indicated.
In one such surgical treatment, the affected disc is essentially removed and the adjacent vertebrae are fused together. In this treatment, a discectomy procedure is conducted to remove the disc nucleus while retaining the annulus. Since the disc material has been removed, a body must be placed within the intervertebral space to prevent the space from collapsing.
In early spinal fusion techniques, bone material, or bone osteogenic fusion devices, were simply disposed between adjacent vertebrae, typically at the posterior aspect of the vertebrae. In the early history of these osteogenic fusion devices, the osteogenic fusion devices were formed of cortical-cancellous bone which was not strong enough to support the weight of the spinal column at the instrumented level. Consequently, the spine was stabilized by way of a plate or a rod spanning the affected vertebrae. With this technique, once fusion occurred across and incorporating the bone osteogenic fusion device, the hardware used to maintain the stability of the spine became superfluous.
Following the successes of the early fusion techniques. focus was directed to modifying the device placed within the intervertebral space. Attention was then turned to implants, or interbody fusion devices, that could be interposed between the adjacent vertebrae, maintain the stability of the disc interspace. and still permit fusion or arthrodesis. These interbody fusion devices have taken many forms. For example, one prevalent form is a cylindrical hollow implant or xe2x80x9ccagexe2x80x9d. The outer wall of the cage creates an interior space within the cylindrical implant that is filled with bone chips, for example, or other bone growth-inducing material. Implants of this type are represented by the patents to Bagby, U.S. Pat. No. 4,501,269; Brantigan, No. U.S. Pat. No. 4,878,915; Ray, U.S. Pat. No. 4,961,740; and Michelson, U.S. Pat. No. 5,015,247. In some cases, the cylindrical implants included a threaded exterior to permit threaded insertion into a tapped bore formed in the adjacent vertebrae. Alternatively, some fusion implants have been designed to be impacted into the intradiscal space.
Experience over the last several years with these interbody fusion devices has demonstrated the efficacy of these implants in yielding a solid fusion. Variations in the design of the implants have accounted for improvements in stabilizing the motion segment while fusion occurs. Nevertheless, some of the interbody fusion devices still have difficulty in achieving a complete fusion, at least without the aid of some additional stabilizing device, such as a rod or plate. Moreover, some of the devices are not structurally strong enough to support the heavy loads and bending moments applied at certain levels of the spine, namely those in the lumbar spine.
Even with devices that do not have these difficulties, other less desirable characteristics exist. Recent studies have suggested that the interbody fusion implant devices, or cages as they are frequently called, lead to stress-shielding of the bone within the cage. It is well known that bone growth is enhanced by stressing or loading the bone material. The stress-shielding phenomenon relieves some or all of the load applied to the material to be fused, which can greatly increase the time for complete bone growth, or disturb the quality and density of the ultimately formed fusion mass. In some instances, stress-shielding can cause the bone chips or fusion mass contained within the fusion cage to resorb or evolve into fibrous tissue rather than into a bony fusion mass.
A further difficulty encountered with many fusion implants is that the material of the implant is not radiolucent. Most fusion cages are formed of metal, such as stainless steel, titanium or porous tantalum. The metal of the cage shows up prominently in any radiograph (x-ray) or CT scan. Since most fusion devices completely surround and contain the bone graft material housed within the cage, the developing fusion mass within the metal cage between the adjacent vertebrae cannot be seen under traditional radiographic visualizing techniques and only with the presence of image scatter with CT scans. Thus, the spinal surgeon does not have a means to determine the progress of the fusion, and in some cases cannot ascertain whether the fusion was complete and successful.
The field of spinal fusion can be benefited by an intervertebral fusion device that can support bone growth material within the intervertebral space, while still maintaining the normal height of the disc space. The device would beneficially eliminate the risk of stress-shielding the fusion mass, and would also provide for visualization of the fusion mass as the arthrodesis progresses.
To address the current needs with respect to interbody fusion devices, the present invention contemplates a osteogenic fusion device that is configured to place as much of the bone growth-inducing material as possible into direct contact with the adjacent bone. In one embodiment, the osteogenic fusion device includes an elongated body having opposite first and second end pieces separated by an integral central element. The central element has a significantly smaller diameter than the two end pieces. The osteogenic fusion device thus forms an annular pocket between the end pieces and around the central element.
In accordance with one aspect of the present invention, a bone growth-inducing material is disposed within the annular pocket around the central element of the osteogenic fusion device. In one specific embodiment, the bone growth-inducing material can constitute a sheet of a pharmaceutically suitable carrier for a bone growth factor, such as a bone morphogenetic protein. In this embodiment, the sheet can be a collagen sheet that is soaked with the BMP and then subsequently wrapped in spiral fashion around the central element of the osteogenic fusion device.
In one feature of the present invention, the osteogenic fusion device can be implanted in a bi-lateral approach. Specifically, two such osteogenic fusion devices can be inserted into prepared bores formed in the endplates of adjacent vertebrae after completion of a discectomy. The spinal loads are borne by the two end pieces that are in direct contact with the adjacent vertebral bodies. Preferably, the osteogenic fusion device has a length sufficient to allow the end pieces to at least partially contact the harder bone at the apophysis of the adjacent vertebrae. With the osteogenic fusion device thus inserted, the bone growth-inducing material is in direct contact with the adjacent vertebral bodies. In addition, bone growth-inducing material can be placed within the bi-lateral space separating the two osteogenic fusion devices. When fusion occurs, a substantial fusion mass is produced that is virtually uninterrupted by the material of the osteogenic fusion device itself.
Several alternative embodiments of the osteogenic fusion device are presented, all retaining the capability of supporting bone growth-inducing material so that it is in direct contact with the adjacent vertebrae. In some embodiments. additional elements of the central element are provided, while in another embodiment, an intermediate piece is provided for further support across the disc space. In one embodiment, osteogenic fusion devices are provided wherein at least one of the opposite end pieces includes a truncated surface. In yet another embodiment, the truncated surface advantageously includes opposite faces, such as opposite edges, that define an entrance to a cutout region. The cutout region is typically defined by the truncated surface and the truncated surface is preferably concave. Such implants are advantageously configured to nest within another fusion device, such as the fusion device of the present invention.
Another embodiment of the present invention provides an implant system including at least two load bearing members as described above adapted to be bilaterally placed between adjacent vertebrae, wherein at least one of the load bearing members has a truncated surface configured to nest within the other load bearing member.
Yet another embodiment of the invention provides an implant system for promoting fusion bone growth in the space between adjacent vertebrae which includes at least first and second load bearing members adapted to be bilaterally placed between adjacent vertebrae, wherein the load bearing members are connected to one another so as to resist lateral separation. In particular, a preferred embodiment provides a first of the load bearing members including a male member, and a second of the load bearing members including a female member. The male and female members cooperate to resist lateral separation of said devices. In another preferred embodiment, the load bearing members can be connected by a connecting member such as a plate spanning the two load bearing members.
In other embodiments of the invention, methods of promoting fusion bone growth in the space between adjacent vertebrae are provided. The methods include providing load bearing members or implant systems as described above, preparing adjacent vertebrae to receive the load bearing members or implant systems in an intervertebral space between adjacent vertebrae and placing the load bearing members or implant systems into the intervertebral space after the preparing step.
The present invention also contemplates an insertion tool and certain modifications to the osteogenic fusion device to accommodate the tool. In one preferred embodiment, the tool is essentially an elongated shank having opposite prongs extending therefrom. The prongs can engage truncated side walls of one of the end pieces. In addition, the opposite end piece can be formed with notches to receive the tips of the two prongs. With this design, the osteogenic fusion device can be a push-in or a threaded type osteogenic fusion device.
It is one object of the present invention to provide an interbody fusion device that allows the greatest possible contact between the adjacent vertebrae and the bone growth-inducing material supported by the osteogenic fusion device. It is a further object to provide such a osteogenic fusion device that is capable of supporting the loads generated throughout the spine without stress-shielding developing bone within the osteogenic fusion device.
Another object of the invention is achieved by features that minimize the radiopacity of the device. This results in a benefit to the surgeon of being able to more readily assess the progress of a spinal fusion.
Yet another object of the invention is to provide an interbody fusion device whereby enough lateral exposure is present to place two large devices side-by-side to distract the disc space and facilitate fusion.
It is yet another object of the invention to provide an interbody fusion device which can be placed closer to another interbody fusion device and which will require no or minimal resection of facet joints.
Yet a further object of the invention is to provide an implant system which is placed in the intervertebral space with minimal retraction of the spinal cord to lessen the chance of neurological complications or damage.